The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Present energy storage is substantially limited by the current chemical battery technology. This is due to both extensive infrastructure development over decades, and wide availability of components and materials in the commercial market. Chemical batteries have high power density, and can easily power most commercial devices for short time periods. However, they cannot withstand the test of time, when storing energy for more than a decade. In addition, chemical batteries suffer from charge leakage, temperature and environment sensitivity, and finite charge cycles.
Radioisotope batteries have the potential to fulfill these technical deficiencies. They are different from chemical batteries, because they are independent, self-containing energy sources using radioisotope decay. They produce consistent power over a widely-varied temperature range, meaning that they are not limited by diverse environmental conditions. Radioisotope batteries energy densities are also several orders of magnitudes greater than current chemical batteries. In general, then, radioisotope batteries have the theoretical ability to remedy deficiencies in current technologies, as well as introducing new operational capabilities, because of higher energy density, thermal and mechanical robustness, and a vastly longer lifetime compared to commercially available chemical batteries.
Isotopes decay through three types of particle emission: beta (electron or positron), alpha (atomic nucleus emission), and gamma (electromagnetic radiation). Beta emitting isotopes are the most appealing candidates for energy sources, as they do the least amount of harm to the semiconductor (converter) and to the environment. Beta-emitters are especially attractive fuel sources for use with sensitive electronics, since their high-energy electron emission has shallow penetration depth through the surrounding material.
Tritium and Nickel-63 have low energy beta emissions, are widely available in the commercial market, and have each been used for several battery prototypes. Tritium is the isotope of choice, since it is the least expensive per kilogram and kilojoule of all beta-emitting radioisotopes, low toxicity and is a low energy beta emitter with a half-life greater than 12 years.
Radioactive decay energy is converted to electrical energy using two main approaches: direct, charge collection, contact potential difference direct energy conversion (DEC), and indirect energy conversion (IDEC) through photovoltaics. The most efficient approach is the DEC configuration. In a two-dimensional perspective, the radioactive source is encapsulated or, in most situations, bonded to another compound called the carrier system. The layer(s) of encapsulated radioactive isotope emit beta particles (electrons) through the carrier system, hitting p-n junctions. Electron-hole pairs (e-h-ps, ehps, or EHPs) are produced in the surrounding semiconductor by the ionization trails of the beta particles. Use of low energy beta particles provide enhanced lifetimes, due to the absence of semiconductor degradations. The configuration can be compact, and can theoretically achieve the highest surface power density of all the energy conversion approaches.
In practice, however, DEC radioisotope batteries have suffered from major setbacks, including energy conversion efficiency, which is dependent on the semiconductor material, beta flux power, and effective density of the radioisotope. The inventive subject matter discussed herein address the latter two setbacks through improvements to the carrier system.
Prior art systems have energy densities that are too low to satisfy power requirements of most electronics. The metal hydrides have the highest specific activity and beta flux power. However, they suffer from numerous disadvantages common to all metal hydrides. First, there is low electron emission depth and beta self-absorption. Second, the known sources are unstable and brittle. Third, tritiated hydrides can ignite in contact with moisture, and with the exception of zirconium tritide and titanium tritide, known tritiated hydrides are pyrophoric. Known hydrides tend to be toxic. Finally, isotopic hydrides exhibit intrinsic leakage of the isotope, which leads to reduced power output, environmental hazard through radioactive contamination, and systemic failure.
Polymers can theoretically address some of the deficiencies of hydrides, but there is little promise for known tritiated polymers. Known tritiated polymers suffer from low effective energy densities, low specific activity, and are not radiation hardened due to weak binding energies. Carbons forms (carbon nanotubes, hydrogenated graphene, and graphane), which are state-of-the-art beta sources, have higher electron range depth and comparable specific surface activity to other hydrides. They could theoretically be used in isotope battery and fuel cell applications. However, there is not much experimental data to determine, if they are feasible on a micro- and macro-scale fabrication. In general, these tritiated carbon forms have to be made into films or foils due to beta self-absorption/stopping, which is dependent on the compound density. The larger systems are difficult to fabricate and heterogeneous with respect to isotope and carrier compound composition. In general, these geometric constraints severely limit the surface power density.
The known carrier systems for tritium are especially problematic for use in powering solid-state electronics, mobile devices, and sensors. The most promising approach is to select a carrier system that has low density, but comparable gravimetric density (wt % 3H) to tritiated metal hydrides. The beta source could be thicker, having greater electron range depth and still allow the beta particles to reach the converter. The figure of merit is the specific surface activity, which is the volume activity factoring in the electron range depth. Also, to further increase the overall power in the device, there must be greater interaction with the carrier and converter surfaces rather than just increasing the radioactivity per volume. If the configuration is planar, the beta source layers could be thick. If the configuration has a higher aspect ratio (e.g. converter is a honey-comb, trench, or pillared structure), the carrier is capable of coating the structure and filling in the open valleys and gaps, due to higher electron range depth.
U.S. Pat. No. 3,934,162, to Adler and Ducommon, describes a nuclear battery arranged in a fashion similar to a galvanic pile, with alternating layers of polymeric insulating material that demarcate “cells” of copper or Cr—Ni support for a beta-emitting radioactive material. Adler and Ducommon and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. Unfortunately, the patent does not provide insight into the nature of suitable radioactive materials or how they may be formulated. U.S. Pat. No. 6,998,692, to Sanchez, teaches a radioactive power source resident in an IC package, but is similarly silent as to what beta emitter would suffice.
United States Patent Application Publication No. 2010/0204408 describes tritiated polymers for use as a radiation source in nuclear batteries having specialized porous semiconductor collectors. The described polymers are produced by tritiation of a polymer, such as poly(vinylacetylene). Due to the polymeric nature of these beta sources, it is necessary to apply these by wetting, using either melted or solvated polymer. It is not clear, however, if the resulting polymeric films are suitable for use conventional semiconductor materials, or how stable such tritiated polymers are.
One might question whether any of Cesium-137, Cobalt-60, Iodine-129, Iodine-131, Plutonium, Strontium-90 (90Sr), Technetium-99, Nickel-63 (63Ni), Phosphorus-33, Promethium-145, Promethium-146, Promethium-147, or Hydrogen-3 (tritium, 3H) would suffice, but each of these elements, per se, would have significant limitations when used in a Sanchez device, making them impractical as an energy source. Most of these beta emitter radioisotopes are not abundant, which in turn considerably increases the $/kJ making it not practical for mass production. And most emit high energy beta particles that could damage semiconductor convertor or other nearby sensitive electronics.
In addition, tritium is a gas at STP, which is difficult to control in a closed volume without a carrier system. The iodide isotopes, for example, are toxic, as are plutonium and strontium. 90Sr has a half-life of 28.8 years and has one of the lowest $/kJ. Yet, it is highly toxic to the environment and living organisms. It has an estimated 18-year half-life when absorbed into living tissue. It is considered a bone-seeker, which means the element or radioisotope will accumulate in the bones. It chemically behaves like calcium and will replace calcium, causing health risks such as bone cancer. Technetium has a half-life of 211,000 years, whereas the half-life of 131Iodine is only 8 days, both of which are impractical for construction of energy devices. The longer half-life radioisotopes, greater than 25 years, will reduce specific power and power density.
Thus, there is still a considerable need to provide a carrier system for a beta emitter that would be cost-effective, energy dense, non-toxic, and practical with respect to semiconductor and other applications.